Solution-processed organic photovoltaic devices (OPV) have emerged as a promising energy technology due to their ease of processing, low-cost, and ability to be fabricated onto light-weight flexible substrates. Polymer based OPV's have by far been the most studied, and power conversion efficiencies (PCE's) above 6% have recently been reported for polymer:fullerene bulk heterojunction (BHJ) devices. On the other hand, solution processed small molecule BHJ devices have received far less attention. Such molecular heterojunctions (MHJ) have several advantages over their polymer counterparts, in that small molecules have well defined structures, are easily functionalized, are mono-disperse, are readily purified, and do not suffer from batch-to-batch variations. Reports of efficient solution processed MHJ devices have recently emerged that have utilized merocyanine dyes, squaraine dyes, isoindigo, and diketopyrrolopyrrole based chromophores as the light harvesting donor component with a fullerene acceptor. PCE's have reached upwards of 4% for such devices. While these results are encouraging, there still exits a need for the development of novel discrete light harvesting materials. Key parameters for effective small molecule donors include having broad and efficient optical absorption that extends into the near-IR region to maximize photon absorption, deep HOMO levels from −5 to −5.5 eV to maximize open circuit voltages, relatively planar structures for high charge carrier mobility, high solution viscosity and solubilizing side chains for solution to film processing. Additionally, it is important that novel structures have facile and highly tunable syntheses to enable rapid and cheap generation of molecular libraries.
The present invention seeks to address the need for improved light harvesting molecules for molecular heterojunction devices by providing novel and advantageous materials for use in such devices.